1. Field of the Invention
The present invention relates to muffler that reduces the exhaust noise of an engine.
2. Description of the Related Art
A conventional muffler reduces the exhaust noise of an engine may include, for example, a casing having a plurality of sound-muffling chambers, and an exhaust pipe disposed to pass through over at least two sound-muffling chambers. The engine exhaust is guided in the muffler by the exhaust pipe and each time the exhaust passes through a sound-muffling chamber the exhaust noise is successively reduced. The more times the exhaust passes through the sound-muffling chamber, the more the exhaust noise is reduced, and the greater is the sound-muffling performance. On the other hand, when the engine is rotating at a high speed, at which the exhaust flow amount increases, an increase in the pressure loss reduces the exhaust efficiency, and affects the engine output.
Given the above, the muffler described in the Japanese Patent Application Publication No. JP-A-2001-885514 provides an aperture in the exhaust pipe, and also provides an valve that opens and closes the aperture. By driving the valve to open and close the valve, the sound-muffling chamber is bypassed in the exhaust. With this muffler, the valve is closed when the exhaust flow amount is small, such as when the engine is operating at a low rpm. The number of times the exhaust passes through the sound-muffling chamber is large and the exhaust noise is reduced, thereby improving the sound-muffling performance.
In contrast, the valve is opened when the exhaust flow amount is large, such as when the engine is operating at a high rpm. The opening of the valve causes the exhaust to flow through a flow passage that is different from when the valve is closed. The exhaust is exhausted from the casing after passing through a fewer number of sound-muffling chambers than when the exhaust flow amount is small. This arrangement suppresses an increase in the pressure loss in the muffler, thereby improving the exhaust efficiency at high engine rpm.
However, if the approach described in the Japanese Patent Application Publication No. JP-A-2001-88514 is applied to a muffler having two exhaust pipes within a casing, such as in a dual exhaust pipe muffler, there is concern regarding the following problem. Specifically, if an aperture and an valve is provided in the wall of each exhaust pipe, when both valves are open, exhaust flowing from both apertures may interfere with each other to cause an abnormal sound. The resulting abnormal sound hinders the sound-muffling effect of the muffler.
The same type of problem can occur in a muffler having a plurality of apertures in one and the same exhaust pipe, and in a muffler having three or more exhaust pipes within a casing.